The present invention relates to a preparation method of high-purity Tl-based superconductor phase of 115K, and the precursor thereof, in particular, to a method of preparation of the Tl-based superconductor by adding oxalic acid as a chelating agent which equals to half the molar amount of total nitrate anions in the required metal nitrates aqueous solution to obtain the superconductor and the composition of the precursor, represented by the formula: EQU (Tl.sub.a Pb.sub.b Bi.sub.c)Sr.sub.2-x Ca.sub.2+x Cu.sub.y O.sub.z,
wherein (a+b+c)=1, 0&lt;x.ltoreq.1 and 2.ltoreq.y.ltoreq.4.
Since the discovery of high temperature superconductors in 1987, there are several superconductor systems with critical temperature (Tc) of above 77K. These include:
(1) Y-Ba-Cu-O system with the onset Tc of about 90-95K: superconductors of this system have the drawbacks of being unstable in air and moisture, and requiring strict conditions for the handling of the material.
(2) Bi-Pb-Sr-Ca-Cu-O system: the onset Tcs of the superconductors are divided into two, i.e., 80K and 110K, depending on the phases thereof.
(3) Tl-Ba-Ca-Cu-O system: the Tc of superconductors in this system is 125K which is the highest record of Tc. The drawbacks of these superconductors are unstability, quick weathering ageing in air and the released highly poisonous Tl vapor during treatment. The production process at high temperature is very dangerous. Although Pb and Bi are introduced to inhibit Tl vapor effectively, the synthesis of the superconductor phase will be reduced owing to Pb and Bi easily react with Ca and Ba.
(4) Tl-Pb-Bi-Sr-Ca-Cu-O system: Bi and Pb are introduced to reduce the amount of poisonous Tl vapor in the system, which leads to the same results as the above system, except that there is no interference phase, such as no production of Ca and Ba. The Tcs of this system are divided into 90K low-Tc phase and 115K high-Tc phase. The Tc may decrease by about 10% to 20% depending upon the processing of the materials. There are difficulties in application of the superconductor produced with Tc approaching 77K of liquid nitrogen, thus it is necessary to produce high-purity superconductor phase with 115K.
In conventional manufacturing processes for high temperature superconductor, three different methods were used. These methods include (1) solid-state reaction, (2) co-precipitation method, and (3) sol-sel solution. However, there are drawbacks in these methods. For instance, (1) solid-state reaction utilizes metal oxides or carbonates mixed and ground by hand or by machine and then sintered. This method will proceed at high temperature and sinter for a long time so as to overcome the defect of insufficient kinetic energy for diffusion of particles. The treatment of materials will result in low material quality and lack of uniformity of particles during the grinding process. Thus, the method is only suitable in a laboratory, not for large scale production. (2) Co-precipitation method is often used in the manufacturing of ceramic superconductors of two or three elements. Because species of high temperature superconductor cations are complicated and the constants of solubility products (Ksp) thereof are different, this method fails to form stable and accurate composition ratio. Moreover, potassium hydroxide (or sodium hydroxide) is employed to adjust the pH value of the solution, which will easily cause the pH value to vary uncontrolably and form ion pollution. The high saturated suspension will be gelated in sol-gel synthesis to form homogeneous precursors. Cations are closely combined with chelating agents in a complex reaction of a gelating process. The most commonly used chelating agents are organic acids, such as acetic acid, oxalic acid and citric acid. The sol-gel synthesis is not considered suitable for Bi- or Tl-based superconductors since Bi salts are not easily dissolved in water, and Tl (+3) compounds are easily reduced to Tl (+1) and lose the characteristic thereof.
Japanese Laid Open Patent No. 1,224,262 discloses the using of strong acid to dissolve Bi salts, and Japanese Laid Open Patent No. 1,219,004 discloses the use of ethylene glycol and EDTA to substitute the simple chelating agents and to purify Bi-based high temperature superconductors. These prior techniques above were provided to overcome the drawbacks of sol-gel synthesis. However, the treatments disclosed above for Bi-based are not satisfactory, for example, the sintering process becomes complicated and involves greater expenses.
Some prior inventions disclosed that a portion of materials was gelated to produce Tl-based superconductors. For instance, Liu et al. in Japanese Journal of Applied Physics, pt II, 28 (12), 1989, L2155-L2157 disclosed that modified citrates were used in gelation to produce high temperature superconductors. That is, Ca-Ba-Cu-O precursors were first made in gelation, and then mixed with thallium oxide in order to proceed with the step of sintering. However, this method does not improve the weaknesses of manufacturing processes because the reaction of Ca-Ba-Cu-O precursor and thallium oxide is the same as that of solid-state reaction.
Kordas et al. in Applied Physics Letters, 57 (14), 1990, 1461-1463 disclosed that the mixed valance state was accomplished by complicated thallium (+1) methoxyethoxide in a sol-gel synthesis in developing the Tl (+3) precursor. But the used organometallic compound cannot be manufactured easily, and the result of sintering is poor.